What's the best formula to use in based on the below - r.e. date =if conditioning

hello what's my best formula for the below 

a              b                 c                     d

my column c is hard keyed, and d is -1 automated, this generates column B's date.

now i can't have column B having a Sunday value, which generates off a =Test'DDDD' function based on column d, so need to autocorrect to a Saturday value when Sunday value are triggered

I can't use column Das it generates a circular reference which i don't want so assume i need to have column b off column c which is hard keyed